Sound system which compensates for variable noise levels



F. A. MORRIS SOUND SYSTEM WHICH COMPENSATES Feb. 22, 1949.

FOR VARIABLE NOISE LEVELS Filed June 13, 1947 NETWORK INVENTOR. FFH/W( H. MORE/5 BY m Patented F eb. 22, ,1949

OFFICE SOUND SYSTEM WHICH COMPENSATES FOR VARIABLE NOISE LEVELS Frank A. Morris, Rochester, N. Y., assignor to Stromberg-Carlson Company, a corporation of New York Application June 13, 1947, Serial No. 754,473

This invention relates to sound reproducing systems and more particularly to sound systems, such as public address or paging systems, for example, adapted for use in areas having variable noise levels as in factories, dining rooms, theaters, schools, and the like.

Sound reproducing systems of the type indicated above may be provided with volume control means. Manual controls have been used to enable ari operator to adjust the output level to take accountr of changes in the noise level which may take place because of variations in the size of an audience in a theater or the number of persons using a dining room or restaurant, for example. However, it is disadvantageous to require the services of an operator merely for the control of the volume of sound reproduction. Moreover, where the sound reproducing devices, such as loud-speakers, are located in diierent remote positions, such as in various parts of a iactory, a single manual control is not adequate.

Sound reproducing systems have been proposed,

wherein the noise level in the speaker area automatically controls the volume of reproduced sound in response to changes in the ambient noise level to provide a signal which is loud enough to be heard, but not objectionably loud when the noise level is low. Heretof'ore, such proposals have been expensive and in many cases, increases in reproduced sound serve toenergize the noise pickup means so that the increase in Volume of sound reproduced changes at a greater rate than desired.

It is an object of my invention to provide a new and improved system for accurately controlling the volume of reproduced sound in accordance with the noise level within the listening area.

Another object of my invention is to provide a new and improved sound reproducingr system having volume control means arranged such that changes in the volume of reproduced sound depend only upon changes in the noise level and are not affected by changes in the volume of desired sound.

In carrying out the objects of my invention, there is provided a suitable source of sound signal to be reproduced, a variable gain amplifier and at least one sound reproducer or loud-speaker, suitable noise responsive means such as a microphone and means for comparing the signal and noise levels, the resultant being utilized to control tl'ie gain of the variable gain ampliers. In the preferred embodiment of my invention, the output from'an analyzing microphone, located in the listening area,a`nd the output of' the ampli- '7 Claims. l(Q1. 179-1) 2, lier, are both rectified. The rectiiied voltages are delayed, as by integration, and then combined in opposition, the resulting voltage being utilized to bias the amplier.

The details of my invention and renements, thereof, as well as further objects and advantages will be apparent from a reading of the following specications and claims in connection with the accompanying drawing which illustrates one embodiment of the principles oi my invention.

Referring to the drawing, there is indicated a source of signal to be reproduced, represented by the numeral l. This source may be a microphone including a suitable speech amplifier, if necessary, a record player or magnetic sound reproducer, the audio system of a radio receiver, a wired sound service, o1' other suitable source. `A suitable variable gain amplier 2 is employed Vto amplify the output of the source l after attenuation in a suitable attenuation network 2l, if such is employed, the amplifier being variable with respect Yto gain in order to permit adjustment of the sound output. The variable gain amplier may comprise a plurality of ampliers, one of which is provided with variable gain. Such an amplifier may employ variable mu tubes, such as Type 6L?, which increases in mu with decreasing negative bias applied thereto.

The output of ampliiier 2 is suitably connected to at least one sound reproducing Vdevice or loudspeaker 3. The number and disposition of speakers depends upon known factors such as the `size of the listening area, the noise level to be expected, etc. The listening area may comprise a single room or space or a plurality of spaces or rooms which may be adjacent or remote from each other in the same building or in separate buildings.

In each area in which it is desired to maintain a predetermined sound level above ambient noise, there is located noise responsive means such as an analyzing microphone or noise pickup means 5, the placement oi which is not critical, n

In order to obtain a suitable control voltage for regulating the gain of amplifier 2, the output of the pickup microphone l may be amplified :by suitable amplifying means 5. The output of the amplilier 5 is utilized to develop a first control quantity such as a direct current voltage representative of the combined noise and reproduced sound levels, as, for example, circuit means represented by the numeral 5 including suitable rectifying means such as the diode rectier l.

In order to prevent rapid variations in the volume of reproduced sound, which might otherwise take place because of the presence of tran- Y 3 sient noises such as breaking dishes, laughter, etc., and also to eliminate the effect of phase differences between the voltages representing direct and picked up sound, the output of the rec- Vtiiier 'I is impressed upon a suitable filter and integrating means including condensers 8 and 9 and resistance I0, the values of resistance and capacity being chosen so that integration is carried out over' a sufiiciently long period of time to avoid the above mentioned undesirable effects, asV several seconds, for example. There is provided an output resistor` I I across the outputcondenser 9, one end of the resistor being grounded and the other being suitably connected to the variable gain amplifier as explained hereinafter.

Inasmuch as pickup means 4 is responsive to both noise and desired sound, means is provided to enable the system to distinguish between sound components and noise components andthereby prevent forcing the system to maximum gain under its own excitation.

In the preferred embodiment of my invention,

there is provided means for establishing a second control quantity such as a direct current voltage representative only of the sound to be reproduced, such as circuit means I2, associated with the output of amp-liiier 2 in such a way that a portion of the output of variable gain amplier 2 may be rectied as by means of diode rectiiier device I3 and passed through filter and integrating means comprising capacitors IiiV and I5 and resistance Iii.4 'Ihe output of the integrating means is impressed across a suitable resistance I'l, resistance I'I being connected in series with resistance I I between a reference potential as ground, for example, and amplifier 2. The resistances II and I'I are so connected in the series circuit that the control quantities may be compared and utilized to control the volume of sound. In the illustrated form of my invention the difference between the voltages is applied as bias to the variable gain ampliiier 2. In the arrangement shown in the drawing, the two bias sources are connected in such a way as tooppose and the resultant voltage is fed to the ampliiier 2. If there is no excitation noise in the listening area, the resultant bias is zero because the outputs of bias circuits 6 and I2 are equal and opposite, but ii there is noise, there is a resulting bias in bias circuit I3, and the control quantities due to desired sound cancel, leaving only the noise component in circuit 6 representing the noise level in the listening area. Hence, the system cannot build up to a level `beyond that necessary to provide the desired degree of audibility of desired signal over noise because changes in the noise levelbut not changes in the desired sound volumecause variation of bias.

It is important that the characteristics of bias circuits 6 and I2`be as nearly the same as possible.

It is known that high frequency noise components affect audibility 'to a greater extent than low frequency noise components. In order to weight the response of the system so that the masking eiects of various frequencies may be taken into account, suitable well-known nonlinear weighting networks I9 and 20 may be interposed between amplifiers 5 and 2 and bias circuits 6 and I2, respectively. Inasmuch as itis necessary that the response of the two biasY systems be substantially the same, a weighting network must be used in both circuits or not at all.

In order to adjust the actual levels of the desired sound and the output of pickup means 4.,

there may be provided suitable attenuating networks as indicated by the numerals 2l and 22, the former being useful to increase the absolute level and the other being utilized to adjust the signal-to-noise ratio. Whether a program is maintained at a constant signal-to-noise ratio or changes disproportionately with variations in ambient noiseis a function of the gain-bias characteristics of the variable gain ampliiier which can be made to have any desired characteristic.

If only one listening area is provided, a single system variable gain amplifier is sufficient, but if the reproducing means 3 are located in separated areas, it may be desirable to provide a separate variable gain amplier for each device 3 or group of devices and a corresponding number of control units.

While I have illustrated and described what I now consider to be the preferred embodiment of the principles of my invention, other embodiments and modications may be made. For example, the comparison or bias circuits 6 and I2 may be differently arranged and different control quantities representing desired sound and combined noise and desired sound may be developed for control purposes, as thermistors, for example. It is intended that the following claims `will cover all such modiiications as fall within the scope and intent of this invention.

What I claim as new and desire to secure by United States Letters Patent is:

1. In a sound reproducing system oi the type comprising a source of sound to be reproduced, at least one sound reproducing means, and means arranged for automatically controlling the volurne of reproduced sound, the last mentioned means comprising means for developing a first control quantity representative of the combined noise and reproduced sound level in the vicinity of said reproducing means, means for establishing a second control quantity representative only of the sound to be reproduced, and means for combining said quantities for control purposes, said last mentioned means comprising resistance means connected between a source of reference potential and said volume control means, said first quantity being impressed across one portion of said resistance means and said second quantity being impressed across another portion of said resistance means.

2. In a sound reproducing system of the type comprising a source of sound to be reproduced, at least one sound reproducing means, and means for automatically controlling the volume of reproduced sound, the combination of means for providing a voltage representing the total noise level in the vicinity of said sound reproducing means, means for providing a voltage representing the sound to be reproduced, means for integrating both voltages over a Yperiod sufficiently long to render the system substantially unafiected by noise .transients or phase diiierences between the voltages representing the noise level and the signal and means for combining the integrated voltages for control purposes.

3. A sound system as dened in claim 1 having attenuating means for permitting adjustment of the amplitude of the first-mentioned voltage.

4. A sound system as deiined in claim 1 having attenuating means for permitting adjustment of the amplitude of the second-mentioned voltage. u i

5. In a sound reproducing system as defined in claim 1, frequency responsive means associated with each voltage establishing means for weighting the frequency response of each establishing means, each of said frequency responsive means having substantially the same frequency characteristics.

6. In a sound reproducing system of the type comprising a source of sound to be reproduced, at least one sound reproducing means, and an amplifier, the combination of means for providing a direct current voltage representing the total noise level in the vicinity of said sound reproducing means, means for providing a direct current voltage representing the sound to be reproduced, means for filtering both voltages, means for integrating both voltages over a period sufficiently long to render the system substantially unaffected by noise transients, voltage divider means connected between said variable gain ampliler and a source of reference potential, one of said integrated direct current voltages being connected across one .portion of said divider means and the other integrated direct current voltage being applied across another portion of said divider means, said voltages being applied in opposition, and means utilizing the difference between the integrated voltages for controlling the output of said amplifier.

7. In a sound reproducing system of a type comprising a source of sound to be reproduced, at least one sound reproducing means, a variable amplier for controlling the volume of sound reproduced, and means arranged for automatically controlling the gain of said amplifier, said last-mentioned means comprising means for deriving a voltage representative of the sound to be reproduced, means for rectifying said derived voltage, ltering and integrating means comprising a resistance in series with said rectifier and a'capacitor shunting said rectifying means on each side of said resistance, an additional resistance shunting the capacitor most remote from said rectifier, means for deriving a voltage representative of the combined noise and program levels, means for rectifying the last mentioned Voltage, ltering and integrating means associated With said second rectifier comprising a resistance in series with said second rectifler and a capacitor shunting said second rectifier on each side of said last mentioned resistance, a second shunting resistance connected across the capacitor most remote from said second rectifying means, and means connecting said shunt resistances in such a manner that the difference between the voltages applied across said shunting resistances is utilized to control the gain of said variable gain amplifier.

FRANK A. MORRIS.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 1,356,403 Pridham Oct. 19, 1920 2,338,551 Stanke Jan. 4, 1944 2,382,848 Baumgartner Aug. 14, 1945 2,392,218 Anderson Jan. 1, 1946 

